Parsing XML in ElementTree in Python3: iterate over children with dynamic tags

I am trying to use ElementTree to read in values from an XML document, but having some issues. For all the cases I have read values into Python before using ET, I have used a specific tag. In the hypothetical example below, how do I access the string values for s1, s2 etc? I've tried a few different ways, but with no luck, and the examples I've found online (so far) use specific tags. I am trying to end up in the position where I can have one list of the child values from parent Id 'A', another list for the child values of parent 'B' etc. Any thoughts? Thanks,

<document>
   <this id="A">
      <s1>"Val_1"</s1>
      <s2>"Val_2"</s2>
      <s3>"Val_3"</s3>
      ... # more values here
   </this>
   <this id="B">
      <s1>"Val_1"</s1>
      <s2>"Val_2"</s2>
      <s3>"Val_3"</s3>
      <s4>"Val_4"</s4>
   </this>
   ... # continues here
</document>